“…An endogenous Na + /K + -ATPase inhibitor is defined as a substance, which should be specific for and have a high affinity for Na + /K + -ATPase (Haupert 1988). There are several reports showing the presence of Na + /K + -ATPase inhibitor from different tissues (Gonick and Saldanha 1975;Louis and Favre 1980;Haupert and Sancho 1979), plasma (Buckalew and Nelson 1974;Hamlyn et al 1982) and urine (Clarkson et al 1979;Cloix et al 1985). These compounds possess wide range of molecular masses.…”